Fix potential exception in KeepOldest split brain strategy (#3272)
Fix potential exception in KeepOldest split brain strategy (#3272)

If remaining and unreachable lists are empty an exception will be
encountered as soon as .First() is executed to try to find the oldest
node
simonlaroche/akka.net,Silv3rcircl3/akka.net,simonlaroche/akka.net,Silv3rcircl3/akka.net